Do not disturb the sleepy thrall....it will be your doom! Lol j/k. Not sure what level you are but that is part of the new dlc. They are around level 20-22. I belive as long as you don' t get to close to that thrall in the corner you will be ok. :)
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Lol well it wasn't even the higher level ones. It was the regular enemies that went invincible. It's part of DLC 1, on Old Russia, Lunar Complex (where you first encounter the hive).